Now, in regards to the complaint of "Oh my god these endings are all so depressing! No matter what Shepard essentially dooms civilization by destroying the relays!" Well let's just stop and think about each ending, shall we?
Paragon Ending: Enslave the Reapers. Many people think this is the Renegade ending because it was the Illusive Man's plan, but in truth it is the Paragon ending. For starters, like with everything in the ME universe, it's color-coordinated: Blue light on the ramp and a blue beam/shockwave released by the Citadel, and as we all know: blue = Paragon. But furthermore, think about what the outcome would be. Yes, the relays are destroyed, but the Reapers still exist. Now, though, they are controlled by Paragon Shepard's benevolent will. As such, it really isn't that far of a stretch to believe that Shepard will turn the Reapers from being the terrifying destroyers of the galaxy to being instrumental in its construction. Given that the Reapers were the ones that built the relays in the first place, they could just as easily do so again.
Renegade Ending: Destorying All Synthetics. Again, contrary to popular belief, this is the Renegade ending (red light, red beam/shockwave, etc). This offers the bleakest outlook for the future as Shepard wipes out an entire race (the Geth) and a close, personal friend (EDI) in order to assure the absolute destruction of the Reapers. With the Reapers destroyed, the secrets to building the relays will be lost. However, the Protheans managed to build the conduit, so it is possible that society could still rebuild the relays, it'll just take a much longer time.
They Lived Happily Ever After Ending: Synthesis. I'd imagine this is the "and the galaxy became a utopian ideal "world" filled with peace from then on out" ending. All life - synthetic and organic - now share the same DNA. I can only imagine that this would lead to advances in technology and the possibility to rebuild society, coexisting with the now pacified Reapers.
All wars require a period of reconstruction and rebuilding once they're over, Shepard grants the galaxy the hope for the future, the purpose to rebuild, and the comfort of peace.